[safety]

Hello, this is my first post to this forum.  Looks like a great site. 

I have a location that uses dry ice for portable coolers on their delivery trucks.  They have been storing the dry ice in their deep freezer (15000 lbs +) and now have some complaints of CO2 exposure symptoms.  They are in the process of renting sampling equipment to measure the levels, and long term if they are going to keep using the dry ice, they will be looking at installing fixed monitors with alarms.   

They have moved a majority of the dry ice out of the enclosed area of the freezer.  And venting the freezer periodically.  They aren't sure exactly why they stored the dry ice in the freezer (-10F), other than that is the area that they do the packing of the portable coolers. I think somebody was thinking it would hold the dry ice longer in the freezer than in the warehouse.  Dry ice manufacturers that I have been too in the past do not have freezers, they simply store it in coolers and ship it immediately.  I do not know of any freezer that can hold the dry ice below -109.3F.

I am interested in calculating the difference in the rate of sublimation at -10F versus in the warehouse at 75F.  I can't find any documented information on this difference, I have found information about the sublimation rate being between 1 and 2% per hour.  The sublimation point of the dry ice is -109.3F.  I would assume that the sublimation rate may be slowed at -10F vs 75F, but is it significantly different? Is there really any significant reason to store the dry ice in the freezer?  The only advantage that I can think of is that the cooling effect of the dry ice may help the freezer and provide some efficiency gains and it may slow the sublimation rate, however there is a distinct safety disadvantage as the dry ice sublimates to gas in the enclosed area.   The physical properties of the dry ice is at this link. http://www.continentalcarbonic.com/dryice/dryicemsds.php

Thanks for any insight or help that anyone may provide!

[Stepan]

Dry ice surface temperature is still -109F no mater is it in a cooler of exposed to ambient air. Evaporation rate is proportional to heat transfer from air to ice surface. When you have the ice in the cooler, the heat transfer in way slower, and therefore the sublimation rate is slower. So, keep it in vented cooler, it will last longer.   

[Stepan]

PS. You can buy cheap disposable CO2 indicator tubes. They are $50/pack of 10, and will be at your place tomorrow. Check for GASTEC products - dosimeter tubes.

[safety]

Stepan, thank you for the comments.  

We did get Draeger diffusion tubes, but they have a bottom operating temp of 0C.  They were indicating about 2.5% with without the 1.3 conversion factor for 0C.  The sampling equipment with a warming tube and probe are showing high levels 2.1% even with only 2100 lbs stored in the freezer and ventilation done 6 days ago.  Little or virtually no air exchange is occuring.

Looks like the bottom line is that it is not safe to store the dry ice in the freezer (300,000 cu. ft) because of the lack of air exchange.  Even one cooler/bunker of 2100 lb.    

What I am really looking to do is to calculate the sublimation rate at -11F versus 75F.  They are keeping the dry ice in coolers/or bunkers both in the freezer and out of the freezer in the warehouse.  Perhaps a controlled experiment is the only way to get what we need, but I was thinking there must be a way to calculate this in theory.

Thanks!

[Stepan]

Dear Safety, the driving force for sublimation is a heat flux which is approximately proportional to temperature gradient dm/dt=K(T2-T1). if we assume that the heat transfer coefficient is approximately the same in the freezer and without it, than in freezer the sublimation rate (Sf) = K((-11)-(-109))= K*99. Sublimation rate in warehouse (Sw): Sw=K(75-(-109))=K*184. Ratio of Sw/Sf=184/99=2. Therefore evaporation rate at 75F is approximately twice faster than evaporation at -11F.

This is very approximate value. In reality it is highly affected by air movement around the dry ice surface. I assume that air velocity in warehouse is higher than the one in the freezer, therefore the sublimation rate in the warehouse will be much higher in the warehouse than in the freezer.   

The best solution is to use high capacity well insulated coolers/bins. In such coolers, the temperature will drop to -109F and they will hold cold much better than the freezer. Excess CO2 from the cooler can be vented out. After that the air quality can be corrected by increasing the air exchange number.

[fledarmus]

In reality it is highly affected by air movement around the dry ice surface. I assume that air velocity in warehouse is higher than the one in the freezer, therefore the sublimation rate in the warehouse will be much higher in the warehouse than in the freezer.   

The best solution is to use high capacity well insulated coolers/bins. In such coolers, the temperature will drop to -109F and they will hold cold much better than the freezer. Excess CO2 from the cooler can be vented out. After that the air quality can be corrected by increasing the air exchange number.

Insulated blankets covering your dry ice would also limit the air movement around your surfaces and lower the sublimation rates without taking up all the space that the coolers would require. Not as efficient but a little more versatile

[Stepan]

Blanket, tarp, whatever you are planning to invest, and whatever you are ready to loose due to insufficient insulation.